The invention relates to an anti-theft protection system for a motor vehicle, and to a method for operating an anti-theft protection system. The system and the method allow access to (e.g., via a closure system), and use of (e.g., via an immobilizer), a motor vehicle only when authorization is verified.
It is believed that electronic, generally radio-based, closure systems are increasingly replacing conventional mechanical closure systems. It is believed that examples of such radio-based closure systems comprise a code transmitter (e.g., an electronic key, ID transmitter, or ID tag) and at least one transmitting and receiving unit in the motor vehicle. It is believed that examples of such code transmitter have a transponder that contains an electronic code, which is interrogated via radio transmission by one or more of the transmitting and receiving units.
It is believed that various non-contact types of transmission are currently in general use in motor vehicle technology. It is believed that examples of low-frequency systems, e.g., in the 125 kHz frequency band, transmit and receive signals via coils. It is also believed that there are high-frequency systems, e.g. at 433 MHz or 868 MHz.
It is believed that, in general, microwave systems or radar systems can be used for transmitting signals. It is believed that when using radar signals, range or distance can be measured using the radar principle. It is further believed that when using microwaves, distance measurements are essentially based on a radar signal being transmitted in the direction of the measurement object, being reflected by the object, and being received as a reflected signal after a delay time. It is believed that the distance to the object, or changes in that distance, can be deduced by evaluating the phase or time difference between the transmitted signal and the received signal. In addition to pulse delay-time measurement, it is believed that frequency-modulation (FM) or correlation methods can also be used for distance measurement. The general radar measurement principles are described, for example, in J. Detlevsen “Radartechnik” [Radar technology], Springer-Verlag, Berlin, 1989.
When such radio frequency (RF) transmission methods are used for transmitting remote-control signals to closure systems or anti-theft protection systems for motor vehicles, it is believed to be very important to know the location of the code transmitter in order to prevent an unauthorized third party, who is not located in the vicinity of the motor vehicle, from gaining access to the motor vehicle.
It is believed that one possibility for distance measurement is evaluating the mean transmitted power of the radio signals. It is believed that this can be done quite well in low-frequency systems, but involves considerable complexity in terms of antenna design and antenna fitment and, frequently, special adaptations to different motor vehicle types.